Generally, it is common to provide multilamp photoflash units which include a plurality of flashlamps each affixed to the electrical circuit of a printed circuit board. The electrical circuit is connectable to an energizing source, which may be of the so called high voltage type having a voltage of about 2000 volts or the low voltage type wherein the voltage is in the range of about 15 volts. Also, the photoflash unit often includes a radiation-responsive normally open (N/O) switch immediately adjacent all but one lamp of the multilamp units. Each of these switches responds to radiation from a nearby lamp in a manner to provide an electrically conductive path rather than the open or non-conductive path prior to exposure to radiation from a lamp.
Of the known problems associated with the above-described normally-open switches and multilamp photoflash units, resistance to humidity and to catastrophic failure due to burn off or blow off are perhaps two of the better known. More specifically, exposure to moisture conditions tends to cause a very erratic behavior of the usual normally-open type switch due to the tendency of the silver source of the composition to prematurely convert to a low resistance value. Also, the compactness of present day photoflash units necessitates a very close spacing of the lamps or radiation source and the radiation-responsive switches. Thus, the tendency for excess radiation or blow off of the switch is of increased concern.
One known attempt to alleviate the above described humidity problems includes the coating of the switch with a moisture barrier compound, such as a lacquer, for example. However, such coating techniques add another step to the process which is obviously undesirable because of the cost in both materials and efficiency of the process.
Further, it is known that the addition of a glass bead filler to the switch composition reduces the tendency toward blow off, and such a composition is described in U.S. Pat. No. 4,080,155 of Sterling. Moreover, an improvement on the above mentioned glass bead containing composition is disclosed in a co-pending application bearing U.S. Ser. No. 021,398, filed Mar. 19, 1979, now U.S. Pat. No. 4,320,440 and assigned to the present assignee. Therein, a filler material such as aluminum dioxide or titanium dioxide is added to the composition in order to inhibit undesired blow off.
Also, it is known to utilize a blended binder of polystyrene and a thermoplastic in order to effect an enhanced resistance to humidity as disclosed in a pending application bearing U.S. Ser. No. 532,090 filed in the name of Michael R. Kling and assigned to the Assignee of the present application. As disclosed therein, although not completely understood, the blended binder appears to achieve a synergistic effect wherein a greater resistance to humidity is effected than is attainable with other known individual binder materials.
Although the above described humidity and blow off protection procedures have been known, it is obvious that an added step to a process in order to provide moisture control is undesirable. Also, the above described addition of a filler material has only been referred to in connection with a high voltage type discharge device wherein a pulse potential having a limited direction is employed.
Also, even though the above-described techniques and procedures have provided enhanced photoflash units and radiation-responsive switches, it has been found that the available structures and apparatus still leave something to be desired in so far as moisture control under humid conditions is concerned. In other words, improved resistance to undesired effects due to moisture is a condition highly desirable in both radiation responsive switches and photoflash units employing such switches.